Keep alive circuit for cold cathode vacuum tubes



B. G. FIRTH Feb. 27, 1962 Filed June 27, 1958 INVENTOR Bap/warp q. fiery ATTO RN EYS nited States Tnepresentiinvention relatesto cold cathode vacuum tubes and more particularly to circuit means for 'such cold cathode tubes that insure rapid restarting of conduction wthereo-fr f' The type of cold cathode tubes with which the present invention is concerned is that in which the cathode is provided with a coating of magnesium oxide in the form ,of microscopic sponge-like structures, the coating being porous and of a finite thickness. When sucha coating is employed in a vacuum tube, electron emission from the coating does not start until some external stimulus is applied to the coating. Once started, however, electron emission builds up and becomes self-sustaining pro- .vidingan electrode at higher potential is provided'within atent 3,023,365 Patented Feb. 27, 1962 but that a current too low to be read on the milliammeter Y employed in the experiments, of the order of one or two microamperes per'square centimeter of coating, was flowing from the cathode to the collector electrode. This current did not cause luminescence of the coating but surprisingly enough seemed to be extremely stable and to be spread over the entire cathode surface. The condition which led to the sharp dropin' cathode current,

from one that caused luminescence to. a current of the order of a microampereor'so, came aboutz'when a large resistance was inserted in' the circuit of the collector-grid.

--When such resistance 'wassuddenly shunted the cathode current rapidly increased to that of operating conditions with-the characteristic luminescence of the coating; Full current strength, from the low almost r'neasurele'ss current, was obtained in this way in a period of time varying the envelope for collection of the emitted electronsfll.

V.arious devices have been employed for initiating electron emission from such type of cathode coating. One such device is a .filament in the envelope which when heated bombards the coating with electrons. Another such device is an electrode mounted within the envelope and provided with a sharp point directed toward the cathode coating, the point emitting electrons by field emission when the electrode is at a high potential with respect to the cathode, say 9( volts or more. Such a starting device is'disclosed in a copending application ofthe appli cant and Albert M. Skellett, Serial No. 697,928 filed November 21 1957, now Patent No. 2,942,146. Other methods for starting" conductionof cold cathode vacuum .--tubes of the above described type are excitation of the ,cathode coating'byiiieans of a Tesla.coil and flooding of the' cathode coating with ultraviolet light.

.the above, known methods or devices, for initiatingconduction involves one or'more disadvantages if the'device is Each of to be restarted continuously or frequently. The heated filament, if used too often, has a tendency to contaminate the coating of the cathode. The high voltage electrode, aimed at a particular spot of the cathode, may also have a deleterionus efiect upon the cathode coating if used too frequently. The same applies under some conditions to excitation by high frequency from a Tesla coil.

It is frequently advantageous in circuits employing a cold cathode vacuum tube tohave the circuit in stand by condition for rapid reenergization when desired. It had heretofore been believed that stand by condition of a circuit including cold cathode vacuum tubes was not possible because of the characteristics of the magnesium oxide coating. But the advantages inherent in being able to rapidly restart conduction in such type of tube were obvious. During experimentation with tubes having a magnesium oxide coated cathode leading toward the development of cold cathode amplifier tubes, it was noted that under certain conditions of grid and anode voltage the cathode current appeared to die out but after raisingthe grid or plate voltage, the cathode current would start again. When the tube is conducting at prescribed voltages of the collector electrode, the cathode current is at least one milliampere per square centimeter of coating and the coating glows a luminous bluish or purplish color. During the experiments referred to above the luminescence of the cathode vanished and the milliammeter connected in the cathode circuit read zero. Investigation of the conditions wherein the cathode current appeared to drop to zero and then to start again led to the finding thatcathode emission had not entirely ceased from five ten thousandths to one thousandth of a second. This compares with a period of one second required for initial starting of the tube when, for-example, the point electrode starter of the said copending application is employed. H

Accordingly the present invention comprises, in comt-bination with a cold cathode vacuum tube of the type defined, a circuit coupled to the collector electrode and provided with a high resistor and means for shunting the same. 7 A further feature of the invention relates to the use of such restarting circuit when the cold cathode vacuum tube is an amplifier, a tetrode or a pentode employing in addition; to the collector electrode, a control grid and anode. It has been found that if the vacuum tube is 'an amplifier it is essential, if the keep alive current is'to be maintained, that the circuit of the control grid of the amplifier be opened prior to opening of the shunt about the high resistor in the collector grid and that when the tube is to be restarted'the high resistor in the collector circuit must jbe shunted befo'rethe control grid is conuected in its'circuit. Accordingly the invention" includes also a circuit in which a sequential double switch in the with an amplifier of the coldic athode type.

collector and controlgrid c quits is provided for use For a better understanding of the lnvention and o'f an embodiment thereof reference may be had to the accompanying drawing of which the single figure comprises a cold cathode vacuum tube and keep alive circuit embodying the invention, the specific cold cathode tube illustrated being a tetrode amplifier.

In the drawing the amplifier is shown as comprising an evacuated envelope 2 within which is mounted in any suitable manner an electrode structure identified generally by the reference numeral 4 and comprising an oval cathode sleeve 6 having a magnesium oxide coating 8 thereon, an inner collector or keep alive grid 10, an outer control grid 12 and an anode or plate 14. The anode 14 is of generally rectangular cross section and in the narrow side walls thereof is an opening 16 to insure adequate exhaust of the electrode structure during processing of the tube. A base 18 of Bakelite or other material is provided for the envelope 2 within which is the usual press or stem upon which the electrode structure is mounted. For simplicity in the drawing the support means for the electrode structure have not been illustrated and the connections or leads to the electrodes have been only diagrammatically indicated.

The circuit for the above described cold cathode tetrode includes a suitable source of potential indicated as' a battery 20 the negative terminal of which is grounded and the positive terminal of which is connected through any suitable load, indicated as a winding 21, to the anode 14 of the electrode structure. The inner or collector grid 10 is connected through a high resistor 22 which is of the order of 20 megohms to a movable tap on a the tube is to be restarted.

potential divider 24 connected across the battery 20.

' an input terminal 28. In accordance with the invention switch means are provided for first shunting the resistor 22 and then closing the circuit of control grid 12 when When the circuit is to be on stand by, the switch first opens the circuit of the control grid and then removes the shunt about resistor 22. The particular switch diagrammatically illustrated in the drawing comprises a relatively long conducting member 30 adapted to bridge contacts 32 and 34 connected to opposite ends of resistor 22 and a shorter conducting member 36 insulatedly coupled to member 30 for movement therewith and adapted to bridge contacts 38 and 40 connected, respectively, to input terminal 28 and to control grid 12. An operating arm 42' is provided for manipulation of the switch elements 30 and 36. To place the circuit in stand by condition the arm is moved in the direction of the arrow 44 to first disconnect contact 40 from element 36 and thereby open the circuit of the control grid and then with further movement in the direction of the arrow to disconnect element 30 from contact 34 to thereby place the high resistor 22 in the circuit of the collector grid 10. To restart the tube, the operating arm 42 is moved in the reverse direction to first shunt resistor 22 and thereafter close the circuit of the control grid.

With the above described tetrode and keep alive circuit, once conduction has been initiated, the tube may be placed in stand-by condition for an indefinite period of time, during which only a minute current will flow between the cathode and collector grid, and no current will flow to the anode 14, irrespective of whether or not the circuit including the load 21 is opened. Immediately upon shunting resistor 22 the cathode coating will glow its characteristic bluish color indicating that the tube is ready for service as an amplifier.

The invention has now been described in connection with a tetrode amplifier. Obviously the same circuit would be employed in the case of a pentode, a tube having an additional grid between the control grid and anode, and a similar circuit except for that part relating tothe control grid would be employed in the case of a diode.

4 The particular amplifier construction diagrammatically illustrated in the drawing is disclosed and claimed in a copending application of Albert M. Skellett and the present-applicant filed concurrently herewith.

The following is claimed:

1. The combination with a cold cathode vacuum tube having a cathode having a porous sponge-like oxide coating thereon of a type adapted to emit a copious self sustaining stream of electrons once emission has been initiated and providing another electrode of higher potential is present for collection of the emitted electrons, and a. collector electrode, of a circuit connected across said cathode and collector electrode, said circuit including a source of direct current potential connected atits low potential end to said cathode and ata point of higher potential to said collector electrode through a high resistor of the order of twenty megohms, and switch means operative to provide a shunt across said resistor, said resistor when not shunted reducing the current in said circuit from several milliamperes to about .one or two microamperes per square centimeter of cathode coating.

2. The combination according to claim 1 wherein said vacuum tube is an amplifier having a control grid and anode and wherein circuitv means are provided for impressing control potentials on said control grid, said switch means being constructed to connect said last mentioned means to said control grid only after shunting of said resistor and to isolate said control grid from said last mentioned means prior to opening of the shunt.

References Cited in the file of this patent UNITED STATES PATENTS 1,988,271 Gerald Jan. 15, 1935 2,104,844 Affel Jan. 1 1, 1938 2,429,451 Cade Oct. 21, 1947 2,504,699 Kluender Apr. 18, 1950 2,591,824 Johnson Apr. 8, 1952 2,616,040 Hansell Oct. 28, 1952 2,656,487 Fullerton Oct. 20,, 1953 2,802,127 Dobischek et al. Aug. 6, 1957 2,840,693 Strauss June 24, 1958 2,842,706 Dobischek'et al. July 8, 1958 

